This invention relates to a local area networks and more particularly to collision detection in a fiber optic local area network.
One of the more interesting problems in the implementation of fiber optic local area networks, designed to service the Ethernet protocol, has been the development of collision detection techniques. J. P. Reedy and J. R. Jones in "Methods of Collision Detection in Fiber Optic CSMA/CD Networks," IEEE Journal on Selected Areas Communications, Vol. SAC-3, No. 6, November 1985, explored a number of collision detection techniques. There are a number of architectures currently under development to implement the Ethernet protocol in a fiber optic local area network. One of the early proposals for implementation of collision detection in a fiber optic LAN configuration was Fibernet II. See R. V. Schmidt, E. G. Rawson, R. E. Norton, Jr., S. B. Jackson, and M. D. Bailey, "Fibernet II: A Fiber Optic Ethernet," IEEE Journal of Selected Areas Communications, Vol. SAC-1, No. 5, pp 702-711, November 1983. The mechanization set forth in this paper is shown in FIG. 1. In FIG. 1, a fiber optic transceiver 10 (optical modem) associated with a user terminal 12 converts the electrical output from an Ethernet controller card 14 to optical format. At a Fibernet II node 16, the optical energy is converted back to electrical format. These signals pass through the node 16 and are imposed on a standard baseband backplane 18. Collision detection is implemented using an amplitude comparison technique on the backplane 18. This is the same technique that is used in the standard baseband coaxial Ethernet configuration. As all collision detection is done by conventional Ethernet hardware in this active node, the confidence level of accurately detecting collisions is very high.
A more commonly used form of collision detection on fiber optic LAN configurations using Ethernet protocol employs a passive star node and smart optical terminal modems. See, M. H. Coden and F. W. Scholl, "Implementation of A Fiber Optic Ethernet Local Area Network," Proc. SPIE (Fiber Optics Multiplexing and Modulation), Vol. 417, April 1983, pp 48-52. A diagram of this configuration is shown in FIG. 2. In this technique, logic contained in an optical modem 20 monitors for variations between its transmitted signal and the return signal after passing through a passive star element 22. This technique closely resembles the collision detection scheme used in the transceiver tap on the baseband coaxial Ethernet implementation. For small passive star node designs this technique works well. However, as the leg to leg variations on the network start to exceed 3 dB or 4 dB the collision detection capability of the logic rapidly diminishes. Such leg to leg variations can be caused by large differential transmission path losses or manufacturing variability in the construction of large star devices.
To compensate for this increased uncertainty in the ability to recognize collisions with variations in network designs, a hybrid star node design has been developed. See the Reedy and Jones reference set forth above. This design employs both a passive star and a number of active supporting components to provide a higher accuracy level in the detection of collision. A diagram of such a mechanization is shown in FIG. 3. In this design, a small amount of optical energy is tapped from each of the input legs on the star. The energy from each of the taps 24 is converted to electrical form by a separate optical receiver 26. The electrical signal from each of the receivers 26 is compared to establish that no more than one optical modem is transmitting at the same time. If a simultaneous transmission of energy is detected, a 10 MHz oscillator 28 is energized. The output of the oscillator 28 is converted to optical format and coupled into one of the input legs on a passive star 30 reserved for this purpose. The star element 30 distributes the signal to each of the optical modems. At the modem, the oscillator signal is detected and passed onto the Ethernet controller card as a collision indicator. This technique has a very high certainty of detecting collisions. However, the mechanization is quite awkward. The taps 24 steal some optical power margin, individual receivers and logic is needed in addition to the passive star element and one of the legs of the star element must be reserved for the transmission of the collision detection signal.
It is therefore a primary object of the present invention to provide a collision detection technique which overcomes the problems known in the prior art.
Yet another object of the present invention is a simple mechanization able to detect collisions with a high level of confidence.
Yet another object of the invention is a collision detection technique requiring little additional logic elements.